The present invention relates to computer systems, in particular, systems that include a continuity module that has been inserted into a memory card slot.
Certain computer systems may employ a serial bus to transmit signals between a memory controller and memory. An example of such a serial bus has been defined by Rambus Corporation of Mountain View, Calif. That bus, often called the Direct Rambus memory channel, enables transmission of high speed, pipelined signals between a memory controller and memory. A memory card or module coupled to the bus may contain a number of high speed DRAMs, which have a Rambus developed architecture. Such memory devices are often called xe2x80x9cRambus DRAMsxe2x80x9d or xe2x80x9cRDRAMs.xe2x80x9d Alternatively, DRAMs that have a different architecture may be coupled to the Direct Rambus memory channelxe2x80x94if accompanied by an appropriate translation mechanism for translating bus compatible signals to signals that are compatible with those DRAMs.
Systems that employ such a serial bus may include multiple sockets for receiving two or more memory modules or cards. On occasion, a system manufacture may not want to install memory cards into each of the socketsxe2x80x94either because the system manufacturer does not want to absorb the additional expense, or because an extra socket was provided to give the user an upgrade option. Because, however, such a serial bus requires signals to travel through all memory devices until terminated at the termination resistors, even sockets that do not receive a memory card must provide a pathway the enables those signals to complete their journey to the termination resistors. Inserting a continuity module in the socketxe2x80x94which includes traces that enable signals to pass through it, but does not include any memory devicesxe2x80x94provides such a pathway. In essence, the continuity module provides a bridge between the memory controller, the memory modules, and the termination resistors.
Certain systems, which include a serial bus and two or more sockets for receiving memory modules or cards, may apply a relatively light capacitive load to the signal lines that extend from the memory controller to the termination resistors. Such a system may, for example, use one socket to receive a memory module that contains only four memory components and use the other socket to receive a continuity module. Because such systems apply such a light capacitive load to the signal line, much of the high frequency portion of fast edge rate signals may be transmitted along the signal line.
The high frequency spectral content of those signals may produce an undesirable amount of signal reflection. When the memory devices used in such a lightly loaded system are RDRAMs, such resonance behavior could augment a weakness in the RDRAM driver, which could cause a single-bit error to occur, when certain frequencies are applied.
One way to address this problem is to simply add more memory to the system. Adding memory adds capacitance to the signal lines, which helps attenuate the high frequency portion of these signals. Adding memory is not, however, an acceptable solution when the system design requires a small number of memory devices. Accordingly, there is a need for an alternative way to attenuate the high frequency portion of high speed, pipelined, fast edge rate signals that are driven along a serial bus used by a lightly loaded computer system. There is a need for such a system that increases the capacitive load applied to a signal line that couples a memory controller to memory devices, without adding memory. The present invention provides such a system.